But it's not a male participant. It's a substance, not a being. And, again, it's balls-all easy just to set cum -yuri in the blacklist (or just cum on its own, if you're so [dis]inclined).
EDIT: And I wouldn't tag a few girls waiting for a reception from an off-screen sperm-donor to be yuri; only if there's no indication of active male involvement and a presence of active female engagement would it seem like yuri. Still not sure if I'd consider one girl spreading another toward the viewer to be yuri or not, though...
Updated by NeverGonnaGive